This invention relates to a method for extracting gas emanating from a landfill. In particular, the invention relates to a method for determining the amount of gas emanating from the landfill and a method for optimizing collection of the gas.
Generally, a landfill is formed by depositing municipal solid waste and many other types of trash in a canyon or pit (or even on flat ground) and depositing soil on top of the trash. Usually, there are alternating layers of trash and soil, one atop another. The waste and soil layers are individually and collectively porous media through which gas may readily flow. The waste itself, including organic compounds such as cellulose, decomposes microbially. At first, this decomposition is aerobic and produces end products which are primarily carbon dioxide and water. After a while, usually ranging from a few weeks to several months, the waste consumes essentially all free oxygen and begins decomposing anaerobically. Then, microbes break down cellulose and other organic wastes and produce methane (CH.sub.4) and carbon dioxide (CO.sub.2) in substantially equal amounts. The methane gas is useful for fuel.
If free oxygen, such as in air, re-enters the waste, decomposition reverts to an aerobic process, and methane production ceases until essentially all of the free oxygen is again consumed. In general, and within limits, the longer the duration of aerobic decomposition, the longer the recovery time for methanogenesis (methane production). Introduction of air not only delays methanogenesis but also consumes trash that might otherwise have been converted to methane. Moreover, when nitrogen mixes with landfill gas, it is very difficult to purify the methane as may be required for some end uses. The normally occurring carbon dioxide can be removed efficiently, but processes required to remove nitrogen (e.g. cryogenics) are very costly. Air also creates a risk of underground fire and will exacerbate existing fires.
As anaerobic gas production continues, the methane concentration increases in the pores of the trash and soil, and the interstitial gas approaches the composition of the gas produced by the microbial cell itself. The mixture of methane and carbon dioxide (hereinafter "landfill gas" or "LFG") migrates within the landfill by diffusion (concentration-gradient driven mass transport) and advection (pressure-gradient driven mass transport). The LFG moves toward, and eventually saturates, soil and rock layers below and to the sides of the trash. The LFG also moves from the trash through the soil cover to the atmosphere. As the atmosphere is essentially an infinite sink for LFG, LFG keeps escaping along this latter route unless collected. As the gas pressure and LFG concentration increase in the surrounding soils, mass transport to those surrounding soils diminishes and after perhaps a few years, becomes negligible.
As methane is useful for fuel, optimal recovery is desirable. This makes proper collection system design and operation important. A perfect collection system is one which extracts all of the LFG but allows no air intrusion into the waste either by advective or diffusive mass transport. In the absence of an impermeable barrier between the waste and the atmosphere, this cannot be achieved. This is true since diffusion of air into the landfill readily occurs as the pressure regime immediately below the landfill-atmosphere interface approaches atmospheric pressure conditions. Without an impermeable liner, a perfect collection system, therefore, might be considered one which collects all of the LFG but which causes no advective air intrusion.
Common LFG collection systems consist of "wells" connected by pipes to a compressor or blower. Wells are normally vertically-oriented pipes installed in the trash (or in soil which is in pneumatic continuity with the trash). The pipes have perforations or slotted sections at the portions disposed in or near the trash. Alternatively, the wells are horizontal trenches or areas filled with gravel. These trenches or areas may be isolated from the atmosphere by a plastic liner or other impermeable barrier.
None of these common LFG collection systems meets either definition of a perfect collection system as they all admit some amount of air. Since this is the case, and in view of the various reasons for excluding air from the waste during the extraction process, a method is needed to optimally tune the collection system.
To collect LFG, the pressure in the well is reduced below that of the LFG in the landfill. The amount of "pull" exerted by the well on the LFG is controlled by operation of the compressor and/or by flow-controlling valves associated with the wells. Reducing the pressure too much will tend to pull air through the soil cover and into the landfill. However, the requisite amount of pull to cause air intrusion will vary with location of the well due to a variety of factors including unknown local LFG generation rates and the heterogeneities of the waste and soil in the landfill. That is, the LFG concentration, gas pressure, and pneumatic permeability of the porous medium around each well are unknown, spatially heterogeneous, and temporally variable. Therefore, it is difficult to extract all, or even most, of the LFG without locally introducing air into the waste.
The process of controlling flow into the wells is known as "tuning." There are relatively few techniques available for this process. One commonly used technique is to collect and chemically analyze one or more gas samples from the wells for relative concentrations of methane, carbon dioxide, nitrogen, and oxygen, or some combination of these gases. When methane concentration is relatively high and nitrogen is relatively low, for example, little or no air may be penetrating the landfill so the tuner increases the LFG extraction rate. When the gas is nitrogen rich and methane poor, when oxygen is in the gas, or when the molecular ratio of carbon dioxide to methane is high signalling substantial amounts of aerobic decomposition, the tuner reduces the extraction rate.
Even assuming that such chemical analysis correctly indicates whether to increase or decrease LFG flow into the well, there is no direct information on how much to increase or decrease flow. Generally, this process is hit-or-miss, especially since the composition of the landfill changes over time. Moreover, changes in LFG composition occur very slowly in response to changes in collection rates. In other words, if the collection rate is increased too much, i.e. sufficiently to introduce air, the resultant reduction in methane and increase in nitrogen in the well may not be detected for several weeks even though air begins to enter the landfill within minutes. If the extraction rate is too low, LFG escapes to the atmosphere, and based on well gas analysis alone, there is no way to tell how much additional gas can be collected by increasing the extraction rate without risking unacceptable atmospheric intrusion.
Another significant problem with LFG collection is determining whether or not the landfill produces sufficient LFG to justify building a gas collection and processing system. Moreover, where a collection system is in place, it is still possible that more gas could be collected if the collection system were to be upgraded. Determining the amount of gas produced can also aid in tuning the collection system, and negotiating energy sales contracts.
Commonly employed testing systems include using a full-scale collection system, a limited collection system with subsequent data extrapolation, waste decay kinetics modeling, laboratory waste decay studies, and tons-in-place estimates. A procedure based upon the cellulose-to-lignin ratio in the waste is sometimes used in conjunction with these other techniques.
Each of these methods suffers from significant drawbacks. For example, using a full-scale collection system is quite reliable but defeats the purpose of determining whether the collection system should be built in the first place. It also tends to be very costly. Where a limited collection system is used, the tester monitors flow rate and methane content of collected gas until they stabilize. This method necessitates extrapolation of the result to the entire landfill, which is very difficult and error-prone due to the heterogeneities of the waste and soil layers. One commonly used extrapolation method, known as the "radius of influence method," involves installing gas pressure monitoring probes radiating outward in one or more directions from a limited number of test wells installed for the purpose. The probes are at substantially the same depth as the slotted LFG intake sections of the wells. In one specific embodiment of this procedure, the tester measures pressures within the probes under non-extraction conditions and then under extraction conditions. In the process, the tester collects landfill gas pressure data from the probes. The tester also determines an extraction rate at which nitrogen begins appearing in the gas and the next lower incremental extraction rate at which there is no nitrogen (the "sustainable extraction rate" ). The pressure measurements so obtained are used to relate the difference in individual probe pressures under extraction and non-extraction conditions to the radial distances of the probes from the wells. The method assumes that at some distance from the well there will be no difference between the extraction and non-extraction probe pressures due to generation of LFG inside that distance sufficient to offset the extraction rate. This distance is taken to be the radius of a circle (the "radius of influence" ), which in turn, is said to define an "area of influence." The total rate of gas production in the landfill is given by the sustainable extraction rate multiplied by the ratio of total landfill area to the "area of influence." This method suffers from many drawbacks and appears to contradict physical laws such as conservation of mass. Because of significant heterogeneities in waste permeability, the approach appears to be unsatisfactory even as an approximation. It can yield total recoverable gas estimates that are incorrect by several orders of magnitude.
In the waste decay kinetics modeling method, the gas generation rate is estimated based on the history of waste disposed at the landfill. The laboratory waste decay study method usually involves taking samples of waste collected during drilling of the wells and attempting to maintain the samples under anaerobic conditions. At a laboratory, the cylinders are placed in a hot water bath and gas generation is monitored for a period of time. In the tons-in-place estimation method, the tester consults reference books or literature to obtain temporal gas yields per unit time and per unit volume or mass of waste in-place, and determines total gas yield based on an estimate of the total amount of waste in the landfill.
In the cellulose-to-lignin ratio method, the tester collects waste samples and determines a concentration of lignin and cellulose, the former being considered a cellulose-associated conservative species and the latter being the primary methane-producing substrate. Based upon concentrations of lignin and cellulose assumed for raw waste, the measured concentrations show what proportion of the original cellulose has decomposed. The tester estimates the total remaining cellulose in the landfill based on the non-decomposed proportions in the sample. The resulting data may be used, for example, to provide decay rate estimates for input to the waste decay kinetics modeling method.
Most, if not all, of these methods suffer from multiple and significant drawbacks including failure to take into account the heterogeneities of the landfill, the absence of proper statistical analysis for providing good confidence interval estimates of LFG production, suspect assumptions about flow and diffusion of gas in the landfill, and possible contradictions of fundamental and immutable physical laws.
In view of the above, there is a substantial need for a more accurate method of estimating LFG production, and a more accurate and faster method for tuning LFG collection systems.